But the go-ahead for the event has been dependent on organisers successfully rehoming a pair of ospreys who have nested on the 1,600-acre estate near Auchterarder for the past five years.

The latest twist leaves the fate of the 21-year-old festival hanging in the balance after the birds flew in to reclaim their old territory.

It is an offence to disturb birds of prey once they have started nesting, so festival organisers have been rushing to complete construction of a new eyrie away from the main staging area and put measures in place to deter the birds from their favoured site.

DF Concerts had originally hoped to remove the osprey nest from Strathallan but were refused permission by the Roberts family, who own the estate.

Osprey nest at Strahtallan - proposed site for T In The Park. Picture: Contributed

However, they have come under fire in recent days for using a large cherry picker, a Saltire flag and a balloon to ward off the birds.

Now police have been called in to investigate after video footage emerged claiming to show the birds attempting to set up home in the old nest.

The ospreys, named Earl and Countess by locals, were first spotted flying around the estate over the weekend.

But video clips taken on Monday and yesterday appear to show one of the raptors landing in the original nest.

The footage was captured by campaigners from Strathallan T Action Group (Stag), who are battling to stop the music festival being staged in the castle grounds.

Conservationists at wildlife charity RSPB Scotland called police to the estate on Monday afternoon after receiving a copy of the clip, which they say shows the nest is now “active”.

A Stag campaigner said: “We are delighted the ospreys are safely back at Strathallan. We have video footage from both yesterday and today, showing they are nesting.

“On our understanding of the law, this means a criminal offence was committed yesterday prior to the removal of the cherry picker.

“We are assisting the police in relation to the authentication of the footage.

“If a criminal offence has indeed been committed, we would question whether it remains an option for the council to grant planning for the festival.”

A DF Concerts spokeswoman confirmed the return of the Strathallan pair, but denied the birds had settled on their breeding site.

“The expert opinion of our ornithologist is that they have not yet nested,” she said.

However, she pointed out that a decision had been taken to withdraw potential disturbance as a precaution.

She added: “The RSPB believes that the previous nest is active and as such close monitoring via the cherry picker has been removed and we will continue to liaise with the relevant organisations to ensure the protection of the birds.

“We will also provide an update when the chosen location of the birds becomes clear.”

Government officials at Scottish Natural Heritage said the latest situation was “a matter for the police”.

They confirmed that advice being prepared for Perth & Kinross Council ahead of a decision next month on whether to grant planning consent for the festival will take account of the “changing situation with the birds”.

The spokeswoman added: “We remain confident that the event will take place at Strathallan.”

Police Scotland confirmed an inquiry was under way after receiving video footage from Stag.

Ospreys were thought to have disappeared from Scotland around 1916, but Scandinavian birds re-colonised the country naturally in 1954 and have been slowly spreading. RSPB surveys suggest there were 250 to 300 nesting pairs in the UK in 2011 – at least 200 of them north of the Border.

T in the Park was forced to quit its former home at Balado, near Kinross, due to safety fears over an underground fuel pipe at the site.

Tickets for the festival, in early July, have been on general sale since February with a weekend pass costing from £184.

This year’s line-up includes the likes of The Libertines, Kasabian, Stereophonics, The Prodigy and, appropriately, Noel Gallagher’s High Flying Birds.